The present invention relates generally to a power transmission used, for example, in a steering system of a motor vehicle and more particularly, to the power transmission which allows transmission of torque from an input shaft to an output shaft, and prevents transmission of torque from the output shaft to the input shaft.
Generally, a rear wheel steering apparatus of a front/rear wheel steering motor vehicle includes a power transmission interposed between an input shaft and an output shaft and for preventing transmission of load input from tires due to road irregularities, cornering force produced upon turning of the vehicle, etc. to a driving gear such as a motor.
Such power transmission is disclosed, for example, in JP-U 6-32802. Referring to FIG. 3, a brief description will be made with regard to this power transmission.
Coaxially and rotatably accommodated in a casing 1 are an input shaft 2 and an output shaft 3, the input shaft 2 having an end with which a sun gear 4 is integrated, the output shaft 3 having an end with which a first drum 5 is integrated. The first drum 5 and the second drum 6 are coaxially integrally coupled with each other by connecting shafts 7. Planetary gears 8 are rotatably supported to the connecting shafts 7 for coupling the first and second drums 5, 6, and are meshed with the sun gear 4. An internal gear 9 is meshed with the planetary gears 8.
A first coil spring 10 and a second coil spring 11, both having the same winding direction, are wound on outer peripheral surfaces of the first drum 5 and the second drum 6, respectively. As for the winding direction, suppose that the first and second coil springs 10, 11 are both wound counterclockwise from the side of an arrow F in FIG. 3 to the front thereof. The first and second coil springs 10, 11 have one ends engaged with end walls of the casing 1, and the other ends contacting a stopper 12 integrally mounted to the internal gear 9 upon set rotation thereof. Specifically, by way of example, when the internal gear 9 is rotated counterclockwise (as viewed from the direction of the arrow F in FIG. 3. Hereafter, the direction of rotation of parts is viewed from the direction of the arrow F) by a set amount, the stopper 12 contacts the other end of the first coil spring 10 to extend the first coil spring 10. On the other hand, when the internal gear 9 is rotated clockwise by a set amount, the stopper 12 contacts the other end of the second coil spring 11 to extend the second coil spring 11.
Integrally formed with the internal gear 9 is a cylinder portion 13 for enclosing outer peripheries of the first and second coil springs 10, 11. The cylinder portion 13 does not contact the first and second coil springs 10, 11 in the initial state, whereas it contacts outer peripheral surfaces of the coil spring 10, 11 when extended by a predetermined amount in the way as described above so as to restrict the amount of extension of the diameter of the coil spring 10, 11.
The power transmission is constructed as described above, so that when the input shaft 2 receives clockwise torque against load of the output shaft 3, the planetary gears 8 rotate on the connecting shafts 7 with rotation of the sun gear 4, which produces counterclockwise rotation of the internal gear 9 meshed with the planetary gears 8 to extend the first coil spring 10. Thus, lock of clockwise rotation of the first drum 5, which is ensured by winding of the first coil spring 10 initially, is released immediately. As a result, the planetary gears 8 revolve around the sun gear 4 to allow transmission of torque that the input shaft 2 receives to the output shaft 3 through the first and second drums 5, 6. On the other hand, when the input shaft 2 receives counterclockwise torque against load of the output shaft 3, the planetary gears 8 rotate on the connecting shafts 7 in the same way as described above, which produces clockwise rotation of the internal gear 9 to extend the second coil spring 11. Thus, lock of the second drum 6 is released to allow transmission of torque to the output shaft 3.
Moreover, when the input shaft 2 receives torque in a predetermined direction, and the output shaft 3 receives heavy load torque in the same direction at the same time, rotation of the internal gear 9 is reversed as soon as torque of the output shaft 3 is input to the first and second drums 5, 6. This releases extension of the diameter of the first coil spring 10 or the second coil spring 11. As a result, rotation of the first and second drums 5, 6 is locked to prevent transmission of torque from the output shaft 3 to the input shaft 2.
As for the known power transmission, when extension of the diameter of the coil spring 10 (11) is not carried out sufficiently by the internal gear 9, which occurs, e.g. when torque that the input shaft 2 receives and load torque of the output shaft 3 have the same direction, frictional engagement and slippage, i.e. so-called sticking slippage, are discontinuously produced between the extended coil spring 10 (11) and the corresponding drum 5 (6). This produces vibrations which may cause wear of an external surface of the drum 5 (6), resulting in a lowering of the power transmission performance. Thus, conventionally, in order to restrain occurrence of the above sticking slippage, lubricating oil is put between the coil spring 10 (11) and the corresponding drum 5 (6).
However, if lubricating oil is put between the coil spring 10 (11) and the corresponding drum 5 (6) in such a way, the following inconvenience occurs. Upon transmission of torque from the input shaft 2 to the output shaft 3, the coil spring 10 (11) on the side of allowing idling of the drum 5 (6) in being extended slightly outwardly by rotation of the drum 5 (6) tends to be extended further outwardly due to a viscous resistance of lubricating oil. If the amount of extension of the coil spring 5 (6) is greater, the outer peripheral surface of the coil spring 10 (11) comes into contact with an inner peripheral surface of the cylinder portion 13 integrated with the internal gear 9. In that state, if the output shaft 3 receives load torque greater than torque that the input shaft 2 receives and having the same direction, for example, the inner peripheral surface of the cylinder portion 13 integrated with the internal gear 9 is frictionally engaged with the coil spring 10 (11) when rotation of the internal gear 9 is about to be reversed as described above. Thus, before the coil spring 11 (10) which has been extended by the internal gear 9 is fully rewound on the drum 5 (6), the two coil springs 10, 11 balance with each other in the circumferential direction through the internal gear 9 and the cylinder portion 13. As a result, the coil springs 10, 11 are not rewound on the first and second drums 5, 6 thereafter, lowering the cutoff performance with respect to torque that the output shaft 3 receives.
It is, therefore, an object of the present invention to provide a power transmission which always stably ensures the cutoff performance with respect to torque that the output shaft receives.